1. Field of the Present Invention
The present invention relates generally to devices for precision motion control, and in particular, to actuated devices for measuring, manufacturing or positioning objects in mesoscale, microscale or nanoscale technologies.
2. Background
Mechanisms or devices for use in positioning, measuring and manufacturing small objects are well known. Such devices conventionally include some sort of actuator device, such as a piezoelectric actuator made from such materials as PZT or, more recently, relaxor materials such as PZN-PT or PMN-PT. Some of the key benefits of piezoelectric actuator devices relative to other actuator device types are their ability to provide a relatively high amount of work while occupying a small volume. Unfortunately, because of how such devices are manufactured, and because of problems associated with the integration of such actuator devices into macroscopically dimensioned mechanisms, such devices produce only a limited displacement range for a given actuator volume, and thus the practical applicability of such mechanisms is limited.
In order to overcome the limited displacement ranges afforded by such mechanisms, some mechanisms amplify the displacement created by the actuator through the use of one or more lever devices to position or move a portion of the mechanism in one or more degrees of freedom. As used herein, motion in a single “degree-of-freedom” shall be understood to mean a single motion along a generally straight line or a rotation about an axis. In general, the term “single-degree-of-freedom” is used to represent a motion that can be defined in terms of a single value relative to a defined coordinate system. This might be a linear displacement along a linear coordinate or an angular rotation about an rotational coordinate (usually referred to as an axis). In practice this restriction is not necessary. For example, a point along a wire of arbitrary shape can be uniquely defined by its distance from one end of the wire and hence only a single value is required to define its location given that the wire represents the coordinate.
By supporting the object of interest on the movable portion of the mechanism (sometimes referred to herein as a “platform”), or integrating it with the platform, the object may likewise be moved or positioned in one or more degrees of freedom as desired. Such designs are desirable in that they provide the ability to position, measure, or manufacture objects with high range-to-resolution ratios.
Generally, mechanisms that utilize only a single lever may be designed for small (i.e., mesoscale, microscale or nanoscale) platforms, but are particularly inherent to mechanical losses, which are typically referred to as fractional loss of motion. Assuming a platform of finite stiffness, the fractional loss is significantly increased as the lever ratios are increased. Therefore, lever mechanisms are generally limited to small lever ratios in order to avoid high fractional losses, and this, in turn, limits the range offered at a particular resolution. A second disadvantage to levered mechanisms is the occurrence of unwanted degrees of freedom deviating from the principal axis of motion of the platform. For example, a single-degree-of-freedom platform translating along a principal axis will commonly generate a yaw and pitch motion as well. In addition, the motion produced at the end of a simple single lever is arcuate, rather than linear, in nature, thus resulting in addition errors. These errors, coupled with the yaw and pitch described above, are all undesirable for most precision motion applications.
One prior art approach to minimizing the effects of platform motion error is the use of symmetrical lever arrangements. For example, U.S. Pat. No. 6,467,761 to Amatucci et. al. discloses a positioning mechanism of a design employing symmetrical pairs of levers to reduce the effects of yaw motion. The design includes a fixed frame and a moving platform connected to the fixed frame by two pairs of lever arms, wherein each lever arm is free to rotate slightly around a flexure, attached to the fixed frame, that serves as a fulcrum for the lever. One end of each lever arm is attached to the platform, while the opposite end of two of the lever arms makes contact with an actuator that is mounted on the fixed frame. When the actuator is activated, the levers rotate around their respective fulcrums, thus moving the platform by an amplified amount (where the amount of amplification is dependent upon the lever ratio). As a result, the platform's width is dependent upon the length of the arms, and the platform size is effectively large for a high lever ratio. Moreover, the platform footprint cannot be any less than the combined length of the symmetrical lever arms. Consequently, a large platform with large lever ratios will result in a low natural frequency of the system.